One of the five distributors that get their water from Viwasupco also said that an announcement of the water quality would have to come from authorities later.

"We have re-supplied water normally. Whether the water is safe enough or not, Hanoi's Department of Health and scientists will make an announcement," said director Nguyen Huu Toi of the Viwaco Joint Stock Company (Viwaco).

The water supply was cut on Tuesday as Viwasupco said it needed to clean its pipes and reservoirs following findings of oil contamination.

Water has been resupplied to its over 147,000 customers in the southwestern part of the capital, including the districts of Thanh Xuan, Nam Tu Liem and Cau Giay. 

However, the administrations of several apartment complexes in Thanh Xuan and Cau Giay Districts have continued to advise their residents against using the water for drinking and cooking.

Another Viwasupco distributor, the Ha Dong Water One-Member Limited Liability Company (Hadowa), has also restored supply to 150,000 customers in Ha Dong District, parts of Nam Tu Liem District and other localities.

Viwasupco had cut service to parts of Hanoi on Tuesday to clean its pipes and reservoirs after its water source was contaminated by used oil, affecting more than 250,000 families with around one million people in the capital city. The company provides 300,000 cubic meters per day to 10 districts in the entire southwestern part of Hanoi.

The Vietnam Environment Administration under the Ministry of Natural Resources and Environment had said Monday that a truck with a loading capacity of 2.5 tons was seen dumping used oil into a mountain creek in Phu Minh Commune, Ky Son District of Hoa Binh Province, a northwestern neighbor of Hanoi, on Tuesday last week.

The creek is on an upstream section of the Da River, the biggest branch of the Red River, which supplies water to Viwasupco's water tanks.

Tests of the polluted water found high levels of styrene, a substance that is probably carcinogenic.

Hanoi authorities have said that people can use the water for other purposes like washing clothes and bathing, but should not drink or use it for cooking.

The city has demanded that Hoa Binh and local police find the truck and the driver that illegally dumped used oil into the environment, but no further information about the culprits has been forthcoming.

Tran Duc Thang, deputy director of the Department of Natural Resources and Environment of Hoa Binh Province, said it has ordered Viwasupco to suspend supply until its water is guaranteed safe. Why the firm has chosen to ignore this order was not clear.

Viwasupco provided no explanation for resuming the water supply, said director Lai Van Thinh of the Ha Dong Water One-Member Limited Liability Company (Hadowa), another distributor.

The other three distributors of water sourced from the Da River are Ngoc Hai Company, Dong Tien Thanh Ha Nam Company and Western Hanoi Development Company (Hadeco).